1. Field of the Invention
The present invention relates to the improvement of electrochemical cells, such as solid electrolyte fuel cells, water vapor electrolytic cells, oxygen pumps, NOx decomposition cells or the like.
2. Description of the Prior Art
The solid electrolyte fuel cells, i.e. solid oxide fuel cells (SOFC) , are roughly classified into two categories: a so-called planar type, and a tubular type. In the SOFC of planar type, a stack for power generation is constructed by stacking alternately the so-called separators and generator layers. In Japanese Patent Application Kokai No. 05-054,897, generator layers comprising a fuel electrode (anode) and an air electrode (cathode), respectively, are fabricated, interconnectors are also fabricated, and then, amultilayered body is fabricated by stacking alternately the generator layers and interconnectors with interposition therebetween of a membrane comprising ceramic powder and an organic binder. The layered body is then heat-treated to bond the generator layers with the interconnectors.
Alternatively, in Japanese Patent Application Kokai No. 06-068,885, is described that a green shaped body for an interconnector is stacked on a green shaped body for a distributor on the air-electrode side, and this stacked body is monolithically sintered to bond the interconnector with the distributor. In this method, between both the green shaped bodies is applied a material having a thermal shrinkage behavior extremely different from those of the green shaped bodies, whereby a stress relaxation layer is formed to mitigate stress between the green shaped bodies. This stress relaxation layer disintegrates into fine pieces during shrinking upon firing, whereby the stress is mitigated.
The present inventors have studied the manufacture of electrochemical cells of such a planar type SOFC. A typical shape of the such type electric cells is shown in FIG. 7a that is a schematic cross-sectional view. In FIG. 7a, a support of this electrochemical cell is the conjoint body 41. This conjoint body 41 is a conjugated body of an air electrode (cathode) 42 with a separator 44. The air electrode 42 has a planar shape. In the separator 44, a plurality of narrow, long banks 44d and 44e are formed on a plate member 44f and grooves are formed between the banks 44d and 44e. With each top face 44c of the banks 44d and 44e, the principal face 42c of the air electrode 42 is bonded. Additionally, 44a is the bottom face of the separator.
The side face 42b of the air electrode 42 is contiguous to the side face 44b of the separator 44 without difference in level. An oxidant gas flow passage 43 has a rectangular or square cross-sectional shape. The end portions of the oxidant gas flow passage 43 are open to the end faces of the separator 44. Additionally, 45 is a joint boundary face. Adense solid electrolyte membrane 19 is formed on the conjoint body 41. In this instance, the main portion 19a of the solid electrolyte membrane 19 is formed on the top face 42a of the air electrode 42. Both sides of the main portion 19a are extended to form extended portions 19b which cover the side faces 42b of the air electrode 42 and further the upper portions of the side faces of the separator 44. Consequently, the oxidant gas flow passages 43 are kept airtight excepting the openings thereof. A fuel electrode (anode) membrane 20 is formed on the solid electrolyte membrane 19.
However, it has been found that the following problems are posed in the electrochemical cells of SOFC utilizing such a conjoint body. Namely, when these electrochemical cells are subjected to repetition of a heating-cooling cycle between a high temperature exceeding 1,000.degree. C. during electric generation and room temperature, there may happen the case where the internal resistance of the whole conjoint body of the electrode 42 with the separator 44 is increased, and when things are at the worst, the electrode may be separated from the separator. Particularly, with a heating-cooling cycle test as the above conducted under conditions considerably severer than actual use conditions, it has been found that, in some conjoint bodies, fine cracks are developed from near the joint boundary between the electrode and separator. As shown in FIG. 7b, a stress is most likely to be generated at the portion 48 of the interface between the electrode 42 and the separator 44.
Alternatively, the same problem as the above has been found also in electrochemical cells other than the SOFC. For example, in the case of high temperature water vapor electrolytic cells, the cells are subjected to the heating- cooling cycle between room temperature and 1,000.degree. C., with starting and stopping of the high temperature water vapor electrolytic cells. Therefore, cells having characteristics which would not be deteriorated even under such severe conditions have been expected.
The task of the present invention is, even when electrochemical cells such as SOFCs or the like are subjected to the repetition of a heating-cooling cycle, for example, from operation at high temperatures exceeding 1,000.degree. C. to temperature fall to room temperature, to prevent the layer separation and crack formation on and around the joint boundary between the separator and electrode so that the increase of the internal electric resistance can be restricted. A further task is to obviate a possibility of developing cracks in the surroundings of the joint boundary of the electrode and separator.